Chop gate and nozzle

ABSTRACT

A shop gate and a nozzle are suitable for being coupled with the bottom floor of a tundish, said nozzle comprising a hollow tube with an axial bore extending from an inlet opening at a first end of the nozzle to an opposite outlet opening at or adjacent the second, opposite end, and comprising a flange extending substantially normal to the axis of the tube and located between said first and second ends, characterized in that the flange comprises a lower, guiding surface facing towards the second end of the tube, said guiding surface being flat and substantially normal to the tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to metal forming lines such as continuous metal casting lines. In particular, it relates to a pouring nozzle design allowing a safe and reproducible emergency sealing of the through bore thereof in case of problem during the casting operation.

2. Description of the Related Art

In metal forming processes, metal melt is transferred from one metallurgical vessel to another, to a mould or to a tool. For example, as shown in FIG. 1 a ladle (11) is filled with metal melt out of a furnace and transferred to a tundish (10). The metal melt can then be cast through a pouring nozzle (1) from the tundish to a mould for forming slabs or to a mould for forming billets or ingots. Flow of metal melt out of a metallurgic vessel is driven by gravity through a nozzle system (1, 111) located at the bottom of said vessel. In particular, the tundish (10) is provided at its bottom floor (10 a) with a nozzle system (1, 100) comprising an inner nozzle portion (4A) embedded in a refractory lining of the tundish floor (10 b) and an outer, pouring nozzle portion (4 b) extending outside of the tundish as best seen in FIG. 4.

In case of emergency, it must be possible to seal the through bore of the nozzle system (1) to interrupt the casting of metal melt. Some tundishes are equipped with a nozzle exchange device. In such devices, the inner nozzle and the outer pouring nozzles are two separate parts, which are mounted on either side of the frame of the nozzle exchange device in tight contact with one another, In case of emergency, the outer nozzle can be replaced by a new one or by a blank plate sealing the through bore of the inner nozzle.

An alternative solution to nozzle exchange devices is to chop off the outer nozzle portion (4 b) of an integral nozzle system (i.e., comprising both inner and outer nozzle portions in one piece) with a chopper or chopping means. The integral nozzle is fixed to the tundish floor by embedding the inner nozzle portion thereof in the refractory lining of the tundish. The outer nozzle portion of the integral nozzle, protruding out of the tundish is engaged in a chopping device (100) comprising a sliding knife having a sharp leading edge resting on a wall of the tubular portion of the outer section of the nozzle and able to slide across the nozzle tubular portion chopping it off by actuation of a pneumatic, hydraulic, or mechanical arm. A blank plate follows the leading edge to theoretically seal the gaping through bore of the upstream section of the nozzle left intact. The problem with such chopping devices is that in order to have a tight contact between the blank plate and the cut off section of the nozzle tubular portion, the edges of the cut need be smooth and straight. This is seldom the case with state of the art chopping devices (100), which generally produce jagged cut sections. Such devices are therefore able to stop the flow of metal melt but rarely to stop it properly allowing metal infiltration and fins around the sealing area and the mechanism.

The present invention proposes a solution for the formation of a tight contact between the edges of the chopped off tubular portion of a nozzle and the plate of the chopper or chopping means of a chopping device. These and other aspects of the present invention are discussed in continuation.

BRIEF SUMMARY OF THE INVENTION

The present invention is defined by the attached independent claims. The dependent claims define advantageous embodiments. In particular, the present invention concerns a chop gate device comprising a frame defining an upper portion, a mid portion, and a lower portion, and further comprising

-   (a) A nozzle comprising a hollow tube with an axial bore extending     from an inlet opening at a first end of the nozzle to an opposite     outlet opening at or adjacent the second, opposite end, and     comprising a flange extending substantially normal to the axis of     the tube and located between said first and second ends, thus     defining a first, inner nozzle portion extending from the flange and     the inlet and a second, outer nozzle portion extending from the     flange to the outlet, wherein the flange comprises a top, clamping     surface facing towards the first, inner nozzle portion, and a lower,     guiding surface facing towards the second, outer nozzle portion,     said guiding surface being flat and substantially normal to the     tube, said nozzle being coupled to the frame such that the first     inner portion extends above the frame upper portion, the flange is     clamped to the frame mid portion by clamping means, and the second,     outer portion extends below the frame lower portion, -   (b) A chopper or chopping means comprising a plate defining a first     and second opposite major surfaces separated by the thickness of the     plate, and a sharp chopping leading edge, said plate being slidingly     mounted in the frame mid portion, such that the first main, sealing     surface thereof is parallel to and contacts the flange guiding     surface of the nozzle, and such that the chopping leading edge can     contact one side of the second outer portion and can be driven     parallel to the flange guiding surface across to the other side of     the second outer portion; and -   (c) A pusher or pushing means capable of forcing the chopping     leading edge through the second outer portion of the nozzle and thus     sever it off from the nozzle flange and first inner portion, such     that the first main surface of the chopping plate seals the bore at     the level of the flange guiding surface.

In order to ensure an excellent, tight and sealed contact between the sealing surface of the chopper or chopping means and the guiding surface of the nozzle flange, the device frame comprises a presser or pressing means resiliently pressing on the chopper or chopping means to press the chopper or chopping means sealing surface against the flange guiding surface or flange guiding means.

The frame of the chop gate device is provided with a clamp or clamping means for fixing the nozzle to the frame. In an advantageous embodiment, said clamp or clamping means are designed to apply a clamping force onto the flange clamping surface and thus fix in place the nozzle to a receiving surface of the frame. The clamping force may be substantially normal to the clamping surface of the flange. In an advantageous embodiment, the clamping surface of the flange is slanted with respect to horizontal, and the clamp or clamping means apply a clamping force substantially normal to said slanting clamping surface. This geometry allows the nozzle to be centered within the frame of the chop gate device by the clamping forces applied on various sides of the nozzle to said slanting clamping surface.

Unlike the state of the art integral nozzles, the present nozzle is coupled to the frame of the chop gate device by clamping thereto the flange of the nozzle. This ensures a perfect alignment of the nozzle with the frame and chopper or chopping means mounted on the same frame. It is only after clamping the nozzle to the frame that the inner nozzle portion of the integral nozzle is embedded in the lining of the tundish.

In an advantageous embodiment, the clamp or clamping means comprise three clamping elements, the centroids of said three clamping elements forming the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore passing by the third summit. Such clamping elements are to be used with a nozzle which clamping surface comprises three separate support rims jutting out and distributed around the perimeter of the tube, which centroid form a triangle which summits match the summits of the triangle formed by the clamping elements.

In order to achieve as clean a cutting surface as possible when chopping the tube off, it is advantageous if the outer nozzle portion comprises a weakening notch at the interface between the flange guiding surface and the second outer portion, leveled with the leading edge of the chopper or chopping means.

The present invention also concerns a tundish comprising a chop gate device as discussed supra fixed to the outside of the bottom floor thereof, such that the first inner portion of the nozzle protrudes through an opening in the floor of the tundish and is embedded within the refractory layer lining the inner walls of the tundish.

Finally, the present invention also concerns a nozzle suitable for being coupled with the bottom floor of a tundish, said nozzle comprising a hollow tube with an axial bore extending from an inlet opening at a first end of the nozzle to an opposite outlet opening at or adjacent the second, opposite end, and comprising a flange extending substantially normal to the axis of the tube and located between said first and second ends, thus defining a first, inner nozzle portion extending from the flange and the inlet and a second, outer nozzle portion extending from the flange to the outlet, wherein the flange comprises a top, clamping surface facing towards the first, inner nozzle portion, and a lower, guiding surface facing towards the second, outer nozzle portion, said guiding surface being flat and substantially normal to the tube, characterized in that, the outer nozzle portion comprises a weakening notch at the interface between the flange guiding surface and the second outer portion. In one embodiment, the weakening notch may extend around a portion only of the perimeter of the outer nozzle portion. In order to obtain an even cleaner cut, however, the weakening notch may extend around substantially the whole perimeter of the outer nozzle portion.

In one embodiment the flange clamping surface (5 a) is slanted, reducing the thickness of the flange (5) away from the tube. The slanting angle may be comprised between 30 and 60° with respect to horizontal, and may be 45°. In an advantageous embodiment the second, outer portion forms a shroud nozzle and the outlet is formed by at least one window, and may be formed by two or four windows opposed two by two and opening to ambient and being distributed around the peripheral wall of the second outer portion adjacent the second end thereof. In an alternative embodiment, the second outer portion forms a pouring nozzle and the outlet opens axially at the second end of said second outer portion.

In a particular embodiment, the nozzle clamping surface comprises three separate support rims jutting out and distributed around the perimeter of the tube, the centroids of said three support rims forming the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore passing by the third summit.

In order to mechanically strengthen the flange of the nozzle some or all the flanges surfaces, bar the guiding surface, are at least partly clad with a metal casing. For a nozzle comprising three rims as discussed supra, it is advantageous that the three separate support rims be part of the metal casing.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention are illustrated in the attached Figures:

FIG. 1: shows schematically a typical continuous casting line.

FIG. 2: shows two embodiments of shroud nozzles according to the present invention. (a) and (b) show a cross section thereof, and (c) shows a perspective view of the nozzle illustrated in (a).

FIG. 3: shows two embodiments of short pouring nozzles according to the present invention. (a) and (b) show a cross section thereof, and (c) shows a perspective view of the nozzle illustrated in (a).

FIG. 4: shows a side view of a nozzle as illustrated in FIG. 3( b) mounted in a chopping device coupled to a tundish: (a) before chopping, (b) after chopping.

FIG. 5: shows a front view of a nozzle mounted on a chop gate device according to the present invention

FIG. 6: shows chopping means suitable for the chop gate device of the present invention.

FIG. 7: shows a nozzle embodiment with three clamping rims.

FIG. 8: shows a top view of the nozzle of FIG. 7 clamped in a chop tube device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen in FIGS. 1 and 2, a nozzle according to the present invention is an integral nozzle in that a single tube forms both inner and outer portions (4 a, 4 b) of the nozzle tube (4) with a single through bore running from an outlet (2 a) opening at the end of a first portion (4 a) forming the inner nozzle, to an outlet (2 b) opening at or adjacent the second portion (4 b) forming the outer nozzle. The inner and outer nozzle portions (4 a, 4 b) are separated by an external flange (5), extending normal to the axis of the tube and comprising a first, guiding surface (5 b) facing towards the outlet (2 b) and an opposite second, clamping surface (5 a) separated from the first surface by the thickness of the flange. The flange runs around the whole circumference of the tube and serves to fix the nozzle in its casting position.

The flange (5) of a nozzle according to the present invention has several functions. First, the flange is used to clamp the nozzle to a matching receiving surface of the frame of the chop gate device (100) by applying a clamping force by a clamp or clamping means (104) to the second, clamping surface (5 a) of the flange. This ensures that the nozzle be perfectly aligned with the frame and sliding chopper or chopping means (101). The clamping forces may be applied substantially normal to the clamping surface (5 a) of the flange. As illustrated in FIG. 5, the clamp or clamping means (104) may be disposed on at least two opposite sides of the flange. It is also possible to have a clamp or clamping means disposed on four sides of the flange, opposite two by two. In an embodiment illustrated in FIGS. 2( a), 3(a), and 5, the clamping surface (5 a) of the flange is comprised of two distinct slanted clamping surfaces, the thickness of the flange (5) reducing away from the tube. The slanting angle may be comprised between 30 and 60° with respect to horizontal, and the slanting angle may be about 45°±3°. The clamp or clamping means (104) apply a clamping force substantially normal to said slanted clamping surface (5 a), which help centring the nozzle with respect to the receiving surface of the frame (cf. FIG. 5).

Second, the flange first guiding surface (5 b) has the function of guiding the chopper or chopping means as it runs across the tube. This is achieved by the first surface (5 b) facing towards the outlet (2 b) which acts as a guiding surface, and as such is flat and substantially normal to the tube.

Third, the guiding surface (5 b) ensures a tight contact between the remaining portion of the nozzle and the chopper or chopping means, thus sealing the through bore of the inner nozzle portion (4 a) and stopping the flow of metal melt altogether. Indeed, the chopper or chopping means (101) comprise a plate suitable for sliding along a plane substantially normal to the axis of the nozzle. Said plate comprises a leading edge (101 c) which is sharp and hard enough to cut the way of the plate through the tube bringing a main, sealing surface (101 b) of the plate vis-à-vis the through bore of the upstream portion of the nozzle still in place. The combination of a straight course of the chopper or chopping means during the chopping operation, and the tight contact between the chopping plate (101) and the guiding surface (5 b) of the flange (5) ensure that in case of emergency, the flow of metal melt can reliably be interrupted altogether at any time by actuation of the chopper or chopping means (101). In order to ensure a tight contact between the sealing surface (101 b) of the chopper or chopping means (101) and the guiding surface (5 b) of the nozzle flange (5), the frame of the chop gate device is provided with a presser or pressing means (105) resiliently pressing on the chopper or chopping means (101) to form a tight and sealing contact between the chopper or chopping means sealing surface (101 b) and the flange guiding surface or flange guiding means (5 b). As illustrated in FIG. 5, the presser or pressing means may be oriented with an angle, and may be about 45° with respect to horizontal, to apply a force substantially normal to two opposite chamfered edges of the chopper or chopping means (101). Since the chopper or chopping means (101) must be able to slide along one direction through the nozzle tube, the sealer or sealing means must apply their force to two edges parallel to the chopping means trajectory. The application of forces at about 45° on two opposite edges of the chopper or chopping means (101) allows not only to ensure a good contact between the sealing surface (101 b) and the guiding surface (5 b), but also to centre the chopper or chopping means (101) during the whole chopping process.

Fourth, the geometry of the flange perimeter may help to control the angular orientation of the nozzle with respect to its central axis. This is particularly important for nozzles having outlets opening laterally on the wall of the tubular portion of the nozzle, as illustrated in FIG. 2. In such cases, the orientation of the outlets may be very important and can be controlled by ensuring that the flange geometry matches the receiving surface of the frame only in the desired orientation. The perimeter of the flange may have a geometry having the same number of axes of symmetries as the allowed positions of the lateral outlet windows. In particular, if the flange perimeter is a quadrilateral, a trapezium ensures a single allowed position, a rectangle allows two different North-South positions of the nozzle and a square allows four equivalent positions (two North-South and two East-West positions). In summary, if the flange must help orienting the nozzle, the flange perimeter should not be circular.

FIG. 2 shows two embodiments of nozzles according to the present invention. Both have in common that the second, outer portion (4 b) of the nozzle is substantially longer than the first inner portion (4 a). They are both provided with four windows (2 b) opening on the side walls adjacent the end of the nozzle portion (4 b). This kind of outer nozzle is called a shroud nozzle and is suitable for continuous casting of billets or bloom. It allows to protect the metal melt (“shroud”) from any contact with air as it emerges from the nozzle through the lateral outlet windows (2 b). In many cases, two opposite, lateral windows only are provided. The orientation of the respective windows is very important for the quality of the cast parts, and can be controlled with the flange geometry as discussed supra.

FIG. 3, on the other hand shows two embodiments of pouring nozzles which are generally shorter than a shroud nozzle and the outlet (2 b) opens in the axis of the tube. Such pouring nozzles are suitable for casting blooms and billets in moulds. The embodiments of FIGS. 2( a)& 3(a) differ from the corresponding embodiments of FIGS. 2( b)& 3(b) in that in the former two, the second, clamping surface (5 a) opposite the guiding surface (5 b) and facing towards the inlet (2 a) is slanted with respect to the latter, whilst it is parallel to the first, guiding surface (5 b) in the latter two. The geometry of the second, clamping surface (5 a) depends on the design of the clamp or means for clamping the nozzle to the chop gate device (100). A slanted second, clamping surface (5 a) allows for a better centring of the nozzle in the device. Both embodiments, however, have in common that:

-   a) the guiding surface (5 b) is flat and substantially normal to the     axis of the nozzle tube, in accordance with the present invention,     and -   b) the tundish nozzle is aligned with respect to the frame of the     chop gate device trough the clamping means and not, as in     traditional designs, aligned with respect to the floor of the     tundish. This innovative concept allows the nozzle and chopper or     chopping means to be systematically in perfect alignment.

The flange, like the rest of the nozzle is made of a refractory material to resist the high temperatures of a metal melt. The flange (5) is at least partly clad with a metal casing for reinforcing it as it is submitted to clamping stresses applied by the clamp or clamping means (104) of the chop gate device. The guiding surface (5 b), however, shall not be clad with a metal casing (22) because in case of actuation of the chopper or chopping means, said surface must form a tight, heat resistant contact with the chopping plate (101). If the guiding surface (5 b) were clad with a metal casing (22) it may contact metal melt and melt itself thus provoking leaks.

In order to promote the formation of a “clean” chopped off surface, the tubular portion of a nozzle according to the present invention must comprise a weakening notch (6) at the interface between the flange guiding surface (5 b) and the second outer portion (4 b). The notch may run around the whole circumference of the tube as illustrated in FIGS. 2( c) and 3(c). It may run as a continuous notch or, alternatively, it may run as an intermittent (e.g., dashed) notch, around a portion only or substantially the whole perimeter of the outer portion of the nozzle.

As illustrated in FIG. 4, a chop gate device (100) according to the present invention comprises a frame defining an upper portion, a mid portion, and a lower portion, a nozzle according to the present invention can be clamped in the frame by clamp or clamping means (104), such that the first, inner portion (4 a) of the nozzle extends above the frame upper portion, the flange (5) is fixed to the frame mid portion by clamp or clamping means (104), and the second, outer portion of the nozzle extends below the frame lower portion, The device (100) further comprises a chopper or chopping means (101). The chopper or chopping means (101) comprise a plate defining a first and second opposite major surfaces (101 a&b) separated by the thickness of the plate, and a sharp chopping leading edge (101 c). The plate is slidingly mounted in the frame mid portion, such that the first main, sealing surface (101 b) thereof is parallel to and contacts the flange guiding surface (5 b) of the nozzle, and such that the chopping leading edge (101 c) can contact one side of the second outer portion (4 b), as illustrated in FIG. 4( a). From said position, which allows casting of a metal melt to proceed, the plate can be driven in case of emergency parallel to the flange guiding surface (5 b) and across to the other side of the second outer portion (5 b), thus chopping the nozzle second outer portion (4 b) of the nozzle, as shown in FIG. 4( b). A tight and sealing contact between the top, sealing surface (101 b) of the chopper or chopping means and the flange guiding surface is assisted by pressing forces applied by presser or pressing means (105) onto the plate of the chopper or chopping means. It is advantageous that the forces are applied on two opposite sides of the chopper or chopping means plate, or with an angle of the order of 45°±3° (although other angles are possible) so that the plate is continuously being centred with respect to the nozzle even when the chopper or chopping means (101) are in motion. As illustrated in FIG. 5, in this embodiment, the plate of the chopper or chopping means must have two opposite edges chamfered to an angle mating the orientation of the presser or pressing means. It is clear that the presser or pressing means (105) must be positioned so as to clear the way to the chopper or chopping means, when the latter is activated.

The plate is usually made of casting steel but it may also be made of a refractory material as, in case of use, the main top, sealing surface (101 b) thereof will seal the through bore of the nozzle extending upstream from the flange (5) and thus be in contact with metal melt. The leading edge, on the other hand, may be reinforced with a hardened material, such as carbides and the like. The guiding surface (5 b) of the flange of the nozzle serve both as guiding surface to ensure that the leading edge (101 c) follows the desired course, but also as sealing surface in matching collaboration with the sealing surface (101 b) following gapless downstream of the leading edge (101 c). Both flange guiding surface (5 b) and sealing surface (101 b) should be made of bare refractory material (i.e., not clad with a metal casing), lest a metal leak would melt a portion of casing cladding such surfaces.

FIG. 6 shows an embodiment of a chopper or chopping means (101) suitable for the chop gate device of the present invention. In the illustrated embodiment, it can be seen that a hole (102) encloses the outer portion (4 b) of the nozzle (see dashed lines indicating the position of the nozzle), with the downstream edge of the hole forming the leading edge (101 c) that will run through and chop the tube in case of emergency. Directly downstream of said leading edge, a sealing plate (101 b) is provided with no gap between the leading edge (101 c) and sealing surface (101 b) to seal the gaping hole of the chopped tube. This geometry decreases the amount of metal melt spillage during the chopping operation to a minimum.

Actuation of the chopping plate is driven by a mechanical, pneumatic, or hydraulic arm (102), able to push the chopper or chopping means (101) from a casting position, allowing the metal melt to flow undisturbed (cf. FIG. 4( a)), to a sealing position, wherein the leading edge (101 c) thereof is driven by a distance, d, to the other end of the nozzle tube, chopping the portion of tube (4 b) extending downstream of the flange guiding surface (cf. FIG. 4( b)). The plate main top, sealing surface (101 b) contacts tightly the flange guiding surface (5 b), thus interrupting the flow of metal melt by sealing the gaping bore (2) at the flange level.

A chop gate device (100) as defined supra is fixed to the outside of the bottom floor (10 a) of a tundish (10). A nozzle as defined supra can then be clamped into position onto the frame of the device (100) by the action of a clamp or clamping means on the flange clamping surface (5 a). The first inner portion (4 a) of the nozzle (1) protrudes through an opening in the floor (10 a) of the tundish thus forming an inner nozzle, which can then be embedded within the refractory layer (10 b) lining the inner walls of the tundish. The refractory layer may comprise a well block for housing the inner portion (4 a) of the nozzle. The flange (5) and second outer portion (4 b) of the nozzle, on the other hand, stand outside of and below the tundish. A tundish thus equipped is safe to use as the casting operation can easily be interrupted in case of emergency and the flow of metal melt stopped almost instantly. This is rendered possible by the use of a nozzle according to the present invention that comprises a specific flange guiding surface (5 b) capable of guiding a chopper or chopping means linearly along a well defined trajectory to chop a portion of the nozzle along a straight plane. On the other hand, the sealing surface (101 b) of the plate of the chopper or chopping means (101) also acts as blank plate, sealing the bore (2) of the nozzle portion still in place, i.e., standing upstream from the guiding surface (5 b), with the advantage that there is no gap between the leading edge (101 c) and the sealing surface (101 b).

In the embodiment illustrated in FIG. 7, the clamping surface (5 a) of the flange (5) comprises three separate rims (30 a, 30 b, 30 c) for wedging the inner nozzle against the frame of chop tube device, the three rims projecting from and being distributed around the perimeter of the tube. The term “separate rims” refers to separated, non-adjacent rims. Each wedging rim has a so-called bearing surface intended to be in contact with the frame of the chop tube device, extending along a substantially horizontal plane, referred to as the bearing plane, and a so-called clamping surface (5 a) to cooperate with the clamp or clamping means of the chop tube device. The so-called clamping surface is arranged facing the bearing surface so that the clamping system and the frame sandwich the wedging rim under the action of the clamping system. The wedging rims are advantageously entirely made of metal and are part of the metal casing of the nozzle. The bearing surfaces or clamping surfaces are advantageously planar. Alternatively, the surfaces may have various shapes, for example, inclined, convex or grooved.

The centroids of the three support rims form the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore (2) passing by the third summit. In other words, the inner nozzle comprises a vertical central longitudinal plane, wherein the three wedging rims are arranged in a Y shape on the periphery of the nozzle, the base of the Y being arranged in the central longitudinal plane and the two arms of the Y being arranged on either side of said plane. The second and third wedging rims have second and third clamping surfaces (5 a), each of said second and third surfaces being arranged on either side of the longitudinal plane and having a centre positioned at an angle between 30 and 45° in relation to the longitudinal plane, with reference to the centre of the inner nozzle. Although not essential, it is advantageous that said second and third clamping surfaces of the nozzle be arranged symmetrically in relation to the longitudinal plane. The first clamping rim comprises a first clamping surface, said surface passing through the longitudinal plane and extending substantially symmetrically in relation to said plane, in a surface included in an angular sector between 14 and 52° with reference to the centre of the inner nozzle.

As illustrated in FIG. 8, a nozzle comprising three rims as discussed supra can be used with a chop tube device according to the present invention comprising three clamping elements (50 a, 50 b, 50 c), the centroids of said three clamping elements forming the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore (2) passing by the third summit, at positions matching the positions of the clamping rims (30 a, 30 b, 30 c) of the nozzle.

Numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described. 

I claim: 1-15. (canceled)
 16. Chop gate device comprising a frame defining an upper portion, a mid portion, and a lower portion, and further comprising (a) a nozzle comprising a hollow tube with an axial bore extending from an inlet opening at a first end of the nozzle to an opposite outlet opening at or adjacent the second, opposite end, and comprising a flange extending substantially normal to the axis of the tube and located between said first and second ends, thus defining a first, inner nozzle portion extending from the flange and the inlet and a second, outer nozzle portion extending from the flange to the outlet, wherein the flange comprises a top, clamping surface facing towards the first, inner nozzle portion, and a lower, guiding surface facing towards the second, outer nozzle portion, said guiding surface being flat and substantially normal to the tube, said nozzle being coupled to the frame such that the first inner portion extends above the frame upper portion, the flange is clamped to the frame mid portion by a clamp, and the second, outer portion extends below the frame lower portion, (b) a chopper comprising a plate defining a first and second opposite major surfaces separated by the thickness of the plate, and a sharp chopping leading edge, said plate being slidingly mounted in the frame lower portion, such that the first main, sealing surface thereof is parallel to and contacts the flange guiding surface of the nozzle, and such that the chopping leading edge is configured to contact one side of the second, outer portion of the nozzle and is configured to be driven parallel to the flange guiding surface across to the other side of the second outer portion; and (c) a pusher configured to force the chopping leading edge through the second outer portion of the nozzle and thus sever it off from the nozzle flange and first inner portion, such that the first main surface of the chopping plate seals the bore at the level of the flange guiding surface.
 17. Chop gate device according to claim 16, comprising a presser resiliently pressing on the chopper to form a tight and sealing contact between the chopper sealing surface and the flange guiding surface.
 18. Chop gate device according to claim 16, comprising a clamp configured to applying a clamping force onto the flange clamping surface and configured to fix in place the nozzle to a receiving surface of the frame.
 19. Chop gate device according to claim 18, wherein the clamp comprises three clamping elements, the centroids of said three clamping elements forming the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore passing by the third summit and wherein the nozzle clamping surface comprises three separate support rims jutting out and distributed around the perimeter of the tube, which centroid form a triangle which summits match the summits of the triangle formed by the clamping elements.
 20. Chop gate device according to claim 16, wherein the outer nozzle portion comprises a weakening notch at the interface between the flange guiding surface and the second outer portion, leveled with the leading edge of the chopper.
 21. Chop gate device according to claim 18, wherein the flange clamping surface of the nozzle is slanted, reducing the thickness of the flange away from the tube.
 22. Tundish comprising a chop gate device according to claim 16, wherein the chop gate device is fixed to the outside of the bottom floor of the tundish, such that the first inner portion of the nozzle protrudes through an opening in the floor of the tundish and is embedded within the refractory layer lining the inner walls of the tundish.
 23. Nozzle suitable for being used in the chop gate device of claim 16, said nozzle comprising a hollow tube with an axial bore extending from an inlet opening at a first end of the nozzle to an opposite outlet opening at or adjacent the second, opposite end, and comprising a flange extending substantially normal to the axis of the tube and located between said first and second ends, thus defining a first, inner nozzle portion extending from the flange and the inlet and a second, outer nozzle portion extending from the flange to the outlet, wherein the flange comprises a top, clamping surface facing towards the first, inner nozzle portion, and a lower, guiding surface facing towards the second, outer nozzle portion, said guiding surface being flat and substantially normal to the tube, wherein the outer nozzle portion comprises a weakening notch at the interface between the flange guiding surface and the second outer portion.
 24. Nozzle according to claim 23, wherein the flange clamping surface is slanted, reducing the thickness of the flange away from the tube.
 25. Nozzle according to claim 23, wherein the second, outer portion forms a shroud nozzle and the outlet is formed by at least one window opening to ambient and being distributed around the peripheral wall of the second outer portion adjacent the second end thereof.
 26. Nozzle according to claim 23, wherein the second outer portion forms a pouring nozzle and the outlet opens axially at the second end of said second outer portion.
 27. Nozzle according to claim 23, wherein the weakening notch extends around substantially the whole perimeter of the outer nozzle portion.
 28. Nozzle according to claim 23, wherein the nozzle clamping surface comprises three separate support rims jutting out and distributed around the perimeter of the tube, the centroids of said three support rims forming the summits of a triangle having two summits on either side of the diameter of the cross section of the axial bore passing by the third summit.
 29. Nozzle according to claim 23, wherein the flange is partly clad with a metal casing with the exception of the guiding surface.
 30. Nozzle according to claim 28, wherein the three separate support rims are part of the metal casing.
 31. Chop gate device of claim 21, wherein the slanting angle is comprised between 30 and 60° with respect to horizontal.
 32. Chop gate device of claim 21, wherein the clamping means apply a clamping force substantially normal to the slanting clamping surface of the flange.
 33. Nozzle according to claim 24, wherein the slanting angle is comprised between 30 and 60° with respect to horizontal.
 34. Nozzle according to claim 25, wherein the outlet comprises pairs of opposed windows. 